The present invention relates to a device for controlling the power factor.
As is well known, the “power factor” in an electric load is defined as the ratio between the actual power absorbed by the load itself and the apparent power applied to it.
The power factor is strictly related to the phase displacement between the current drawn by the load and the voltage applied to the latter by the power source: if the drawn current is completely in phase with and has the same waveform as the applied voltage, the power factor will be equal to 1; if, on the other hand, voltage and current are out of phase and/or have different waveforms, the power factor will be less than 1.
A power factor with a value of 1 (or close to 1) is desirable both from an energy standpoint (energy efficiency is maximized) and for reasons tied to distortion (the voltage waveform will not be substantially distorted by the load).
In many applications it is actually not possible to achieve a power factor equal to 1; in particular, in applications such as inverters, motor control or converters for low voltage that operate from a DC bus, the power factor may be much less than 1.
The above-mentioned circuits are typically provided with a bridge rectifier and an electrolytic capacitor connected to the bus and serving to convert the AC input voltage into an adequately filtered DC voltage.
In order to improve the power factor (i.e., to increase it so as to bring it as close as possible to a value of 1), correction circuits are installed between the mains power supply network and the load. These circuits enable a sufficiently high power factor to be obtained (e.g. 0.8–0.9).
Among the power factor correction circuits available today there are systems consisting of a digital controller (typically a DSP) associated with analog power modules.
The digital controller measures all the necessary parameters relating to the supply voltage/current and output voltage/current and generates appropriate command signals that are transmitted to the power modules, so that the latter can deliver power to the load while maintaining a high power factor. Furthermore, the digital controller transmits commands to all modules at the same frequency and controls their synchronization.
A drawback of this type of circuit emerges if one considers the costs of setting it up (DSPs are notoriously expensive devices) and the need to modify the digital control whenever it is necessary to adjust maximum output power by changing the number of modules.
At the present state of the art, an alternative to the circuits briefly described above may be a completely analog-type circuit.
The operating limits of this second type of correction circuit become evident considering that for any change that needs to be made, the hardware making up the circuit itself must be adapted. Moreover, it is very difficult to implement topologies with several parallel power modules.
In other words, as the device is of a completely analog type, it is not possible to substantially modify any function without operating directly upon the electronic circuitry, replacing some components or setting them in a particular manner.
In addition to the above drawbacks, analog implementation of auxiliary control functions (relating, for example to overvoltage conditions or, more generally speaking, to situations where the system is not working correctly) is extremely complex, costly and above all offers little flexibility.